1. Field of the Invention
The present invention relates to a reproduction device to reproduce information in the recording tracks of a recording medium, and particularly to a reproduction device having tracking control means.
While such devices may reproduce information from a magnetic recording medium or an optical recording medium, this specification refers mainly to an embodiment of devices that reproduce information from a magnetic recording medium. However, the present invention is not limited to reproduction from a magnetic recording medium.
2. Description of the Prior Art
Heretofore, it has been difficult to have the head of a magnetic disk reproduction device or a magnetic tape reproduction device accurately face-contact, i.e. come into face to face contact, with a recording track. This is the case because of the accuracy required in positioning the magnetic head, eccentricity of the magnetic disk, and errors in the tape feed, etc. Therefore, it is desirable to use some form of tracking control means.
Detecting the direction of deviation from a track on the basis of only a head's reproduction level is especially difficult in a magnetic disk reproduction device. This will be evident from an example of a device that reproduces video signals from a magnetic disk that has video signals of one frame or one field recorded on a number of respective tracks.
FIG. 1 shows a recording pattern on a magnetic disk 1. As shown in the drawing, odd number fields of video signals are recorded on tracks A, while even number fields of the same are alternately recorded on tracks B. Video signals are frequency modulated and recorded both on tracks A and B. Guard bands "g" between the tracks A and the tracks B prevent crosstalk. An annular track A or B has video signals of one field recorded thereon. Both in recording and in reproducing the disk 1 rotates, for example, with a field frequency of NTSC TV signals (3,600 rpm). At the time of recording, a track width TW, a track pitch TP, and a guard band width GW are preset, and the head position is controlled at the time of recording to record according to a given format.
Conventionally, a so-called combination head CH having two gaps GA and GB shown in FIG. 2(a) is used to reproduce video signals from such a magnetic disk. Gap distances of the gaps GA and GB of the combination head CH are set equal to the track pitch TP, and gap widths HA and HB are also set equal to the track width TW. The heads HA and HB read out video signals of the odd number field tracks A and the even number field tracks B in turn to reproduce one static picture. However, when the heads deviate from the tracks as shown in FIG. 2(b) because of errors in a head positioning mechanism, etc., their reproduction outputs will be damped. This makes it necessary to correct the positional deviation of heads from the tracks. However the reproduction output levels in heads with the head shapes shown in FIG. 2 will be low, and the direction of deviation of the heads cannot be detected.
To overcome this problem, a so-called Dither control system serves to vibrate the magnetic heads at a prescribed frequency and transverse to the direction of the tracks, and detects the direction of deviation of the tracks on the basis of the shift deviation of the heads and change in the detected output. While such a control system can detect the direction of deviation, a complicated control circuit is required. At the same time the head vibration creates jittering in the time base. Thus it is not especially suited for reproduction of the video signals, etc.